The present invention relates to control circuitry actuable by a plurality of inductively coupled, variably tuned keying circuits. In the circuits disclosed in U.S. Pat. Nos. 3,624,415 and 3,628,099, both in the names of Carl E. Arkins and Arthur F. Cake, keying circuits requiring that the correct value of resistance in an external keying circuit be connected are used to actuate a keyable control circuit. In U.S. Pat. No. 3,723,967 in the names of Carl E. Atkins and Paul A. Carlson, a single inductively coupled tuned keying circuit absorbs energy from the radio frequency tank circuit of a free-running oscillator operating at the frequency to which the keying circuit is tuned.
Radio frequency detection circuits detect the reduction in energy remaining in the oscillator and thereupon produce a control signal. The absorption method of keying exhibits the disadvantage that a piece of magnetic ferrous metal could also absorb sufficient energy to cause keying.
In U.S. Pat. No. 3,842,324 a single external keying circuit includes a diode having a sharply variable junction capacitance when subjected to varying conditions of bias. When the keying circuit is exposed to radio frequency energy of the correct frequency, the net capacitance in the keying circuit due to variations in that the diode junction capacitance oscillates at an audio frequency rate, and consequently, varies the amount of absorption of radio frequency energy from the keyable control circuit at the same audio frequency rate. The keyable control circuit connects an enable signal to the load only upon detecting this modulation. Consequently, the diode-modulation method in this invention avoids triggering from mere absorption alone.
The prior art suffered from the fact that only a limited number of discrete radio frequencies could be used with practical circuits. Consequently, the number of different "keys" which could be produced with single-channel devices was limited.